1. Field of the Invention
The present invention relates to a power supply unit employing a DC--DC converter which can be used in portable electronic equipment such as note-book type personal computers and the like.
2. Description of the Related Art
In small electronic devices using battery cells, the power supply voltage drops as the cells are discharged. In order to maintain the voltage used in electronic equipment constant, therefore, use is made of a DC--DC converter which converts the cell output to a predetermined value to stabilize the output voltage. Such a power supply device can be classified into the one of the voltage-dropping type in which a voltage higher than the voltage used in the equipment is supplied from the cells and is dropped through the DC--DC converter to a voltage that is used in the equipment and the one of the voltage-boosting type in which a voltage lower than a voltage used in the equipment is increased by the DC--DC converter to a voltage used in the equipment, depending upon the relationship between the voltage of the cells mounted in the electronic equipment (or the voltage of the AC adapter) and the voltage used in the electronic equipment.
Whether the electronic equipment employs the power supply of the voltage-dropping type or of the voltage-boosting type is determined depending upon the power consumption of the equipment, the operation time that must be possible without renewing the cells, the size of equipment, the weight of equipment, etc.
A related art will be described below, in detail, with reference to the accompanying drawings.
FIG. 1 is a diagram illustrating a conventional power supply unit which is provided with a DC--DC converter of the pulse width modulation (PWM) type, wherein reference numeral 200 denotes a DC--DC converter, 201 denotes battery cells, 202 denotes a control signal generator that generates signals for controlling the turn-on and turn-off of a transistor Tr1, symbol Vin denotes an input voltage, Vo denotes an output voltage, C1 denotes an input capacitor for removing power supply noise, Tr1 denotes a switching transistor for controlling 10 the voltage, L1 denotes a choke coil, D1 denotes a diode, C2 denotes a smoothing output capacitor, R1 and R2 denote voltage-dividing resistors which detect a change in the output voltage and produce an output voltage detect signal EIN, R3 and R4 denote voltage-dividing resistors for forming a reference voltage to generate a stable operation signal DT for the control signal generator, and C3 denotes a capacitor which stabilizes the operation when the DC--DC converter is started, i.e., increases the level of stable operation signal when the power supply is started in order to gradually increase the high-level period of a control signal output by a comparator 211 (see FIG. 2) in the control signal generator 202.
In the constitution of FIG. 1, the control signal generator 202 generates a signal for turning on or off the transistor Tr1 that controls the output voltage. When the transistor Tr1 is turned on, energy is supplied from the cells 201 to the choke coil L1 (hereinafter referred to as coil L1) and is stored therein. Due to the energy stored when the transistor Tr1 is turned off, a fly-back current flows through coil L1--capacitor C2--diode D1 to produce an output voltage Vo across the voltage-dividing resistors R1 and R2. The control signal generator 202 generates a control pulse having a small duty ratio when the output voltage Vo is higher than the reference voltage and generates a pulse signal having a large duty ratio when the output voltage Vo is lower than the reference voltage. Accordingly, the transistor Tr1 is turned on for a short period of time when the output voltage Vo is high and is turned on for a long period of time when the output voltage Vo is low, so that the output voltage Vo is stabilized.
FIG. 2 illustrates a conventional control signal generator, wherein reference numeral 202 denotes a control signal generator and 210 denotes an error amplifier (ERR-AMP) which compares an output voltage detect signal EIN with a reference voltage Vref and outputs a signal ERO. The signal ERO increases with an increase in the output voltage detect signal EIN and decreases with a decrease in the output voltage detect signal EIN.
Reference numeral 211 denotes a comparator (PWM-COMP) which compares an output ERO of the error amplifier 210, a stable operation signal obtained by dividing the voltage Vref of a reference cell 214, to a triangular wave output from a triangular wave oscillator 212, that will be described later, and generates a control signal CPO. Reference numeral 212 denotes the triangular wave oscillator which generates triangular waves, and 213 denotes a driver which amplifies and outputs a voltage that is output from the comparator 211. Symbol EIN denotes an output voltage detect signal, ERO denotes an output signal from the error amplifier 210, CPO denotes an output signal from the comparator 211, DT denotes a stable operation signal, Vref denotes a reference voltage of the error amplifier 210, and R3, R4 and C3 denote items that were described above.
In the constitution of FIG. 2, the comparator 211 compares the triangular wave output from the triangular wave oscillator 212 output ERO from the error amplifier 210, to the stable operation signal, and generates a high-level signal (CPO) during a period in which the voltage of the triangular wave is larger than either the output ERO of the error amplifier 210 or the stable operation signal, whichever is larger (the transistor Tr1 is turned on when the signal CPO is at a high level). The driver 213 amplifies the output CPO of the comparator 211 and feeds it as a control signal to the transistor Tr1.
FIGS. 3A to 3C are diagrams illustrating the operation of a power supply unit, wherein ERO denotes an output of the error amplifier and DT denotes a stable operation signal. The threshold value is a voltage above which the comparator 211 produces a high-level output. When a triangular wave having a voltage larger than the threshold value is input, the comparator 211 operates as described below. Symbol CPO denotes an output signal of the comparator. FIG. 3A represents a case where ERO&gt;DT. Since the output ERO is larger than the stable operation signal DT, the comparator 211 outputs a high-level signal (signal CPO of the high level) during a period in which the triangular wave is larger than the threshold value and the output ERO. That is, the comparator 211 outputs the signal CPO at the high-level during the period from time t0 to time t1 and during the period from time t2 to time t3.
FIG. 3B represents the case where ERO&gt;DT as in the case of FIG. 3A. The triangular wave becomes larger than the stable operation signal DT and the output ERO during a period from time t0' to time t1' and during a period from time t2' to time t3'. During these periods, a high level signal CPO is output from the comparator. The period in which the triangular wave is larger than the output ERO becomes longer than that of the case of FIG. 3A and, hence, the period in which the output CPO has a high level is lengthened.
FIG. 3C represents the case where DT&gt;ERO. Since the output ERO is larger than the stable operation signal DT, the comparator 211 outputs a high-level signal (signal CPO of the high level) during a period in which the triangular wave is larger than the threshold value and the stable operation signal DT. That is, the comparator 211 outputs the signal CPO of the high level during the period from time t0" to time t1" and during the period of from time t2" to time t3".
FIG. 4 illustrates another conventional power supply unit, wherein reference numeral 220 denotes a DC--DC converter, 221 denotes battery cells, 222 denotes a control signal generator, Vin denotes an input voltage, Vo denotes an output voltage, C1 denotes an input capacitor for removing noise, Tr1 denotes a switching transistor for controlling the output voltage, L1 denotes a choke coil, D1 denotes a diode, C2 denotes a smoothing output capacitor, R1 and R2 denote voltage-dividing resistors that detect a fluctuation in the output voltage and form an output voltage detect signal EIN, R3 and R4 denote voltage-dividing resistors for forming a reference voltage to generate a stable operation signal for the control signal generator 222, EIN denotes an output voltage detect signal, DT denotes a stable operation signal, Vref denotes a reference voltage of an error amplifier 210 (see FIG. 2) in the control signal generator 222, and C3 denotes a capacitor for stabilizing the operation when the DC--DC converter is started.
In the constitution of FIG. 4, an electric current flows into the coil L1 and into the capacitor C1 while the transistor Tr1 is turned off, and energy is stored therein. Due to the energy that is stored during the period in which the transistor Tr1 is turned off, a fly-back current flows through the circuit of diode D1--capacitor C2--transistor Tr1, and an output voltage Vo is obtained. The operation of the control signal generator 222 is the same as that of the case of FIG. 1 and is not described here.
In the constitution of FIG. 1, the input voltage Vin is applied to the coil L1 and to the capacitor C2 while the transistor Tr1 is turned on. When the transistor Tr1 is turned off, the fly-back current flows through the circuit made up of diode D1, coil L1 and capacitor C2 due to the stored energy, and a smoothed output voltage Vo is produced.
Here, the output voltage Vo is given by the following equation, EQU Vo=Vin.times.Ton/(Ton+Toff)=Vin.times.Ton/T
where T is a period of the triangular wave, Ton is a period in which the transistor Tr1 remains turned on, and Toff is a period in which the transistor Tr1 remains turned off.
By controlling the duty ratio of a pulse signal output by the control signal generator 202, the output voltage Vo can be set to be constant despite a change in the input voltage Vin.
As represented by the operation of FIGS. 3A and 3B, when the output voltage has changed toward the higher side, the output ERO of the error amplifier 210 increases and the duty ratio of the signal CPO decreases. When the output voltage has changed toward the lower side, the output ERO of the error amplifier 210 decreases and the duty ratio of the signal CPO increases. When the output voltage has changed toward the higher side by controlling the duty ratio, the output voltage Vo can be reduced and when the output voltage has changed toward the lower side, the output voltage Vo can be increased.
Considered below is the case where there is no stable operation signal in the above-mentioned operation. When the output voltage Vo is close to zero due to a drop in the output voltage at the time of starting the power supply or being caused by a sudden change in the load, the error amplifier 210 produces an output ERO which is zero. In such a case, the signal CPO acquires the high level for nearly the whole period of one cycle, whereby an excess current may flow into the transistor Tr1 and damage it. In order to prevent this, a stable operation signal is input to the comparator 211 so as to become larger than the threshold value. Even when the signal ERO is close to zero, therefore, the output CPO of the comparator 211 is maintained at the low level until the voltage of the triangular wave exceeds the stable operation signal, thereby to limit the current that flows through the transistor Tr1.
When the output voltage Vo is smaller than the stable operation signal (such as when the power supply is started), the duty ratio of the signal CPO output from the comparator 211 is expressed as described below (duty ratio of the period in which the switching transistor Tr1 is turned off). The output of the comparator 211 has a low level during a period in which the stable operation signal is larger than the voltage of the triangular wave. Therefore, when the voltage of the stable operation signal is denoted by Vdt,
Duty (off)=100.times.(Vdt-minimum voltage of triangular wave)/amplitude of triangular wave (%), EQU Vdt=Vref.times.R4/(R3+R4)
The average input current Iin flowing into the coil L1 becomes equal to the product of the output current Io and the duty ratio of the transistor Tr1, and is given by, EQU Iin = Io .times. Ton/T
Moreover, a maximum current Ipeak flowing into the coil L1 is determined by the inductance of the coil L1, input voltage Vin and output voltage Vo, and is given EQU Ipeak=Ton.times.(Vo -Vin)/L
where L is the inductance of the choke coil.
As described above, the output current in the DC--DC converter of the PWM control system is determined by the ratio of Ton to Toff. On the other hand, when the input voltage Vin to the DC--DC converter is much larger than the output voltage Vo, an excess of current flows into the choke coil L1 and through the switching transistor Tr1 causing the operation to become unstable. In order to stabilize the operation even in such a case, therefore, the period Ton (duty ratio) in such a state must be shortened. When the input voltage Vin cannot be increased compared with the output voltage Vo, on the other hand, a sufficient current is not obtained when the period Ton is short. Therefore, the period Ton must be lengthened.
An excess current is generated when the DC--DC converter is started. The output voltage Vo is 0 V at the start time, and an excess current (inrush current) flows through the transistor Tr1 since the difference between the input voltage Vin and the output voltage Vo is large. To prevent the large inrush current, a method can be employed to temporarily shorten the period Ton at the start time. This is called a mild start in which the voltage of the stable operation signal is increased at the start time to shorten the period Ton, the stable operation signal is gradually lowered and the period Ton is gradually lengthened. For this purpose, in FIGS. 1 and 4, provision is made of the capacitor C3, and the voltage of the stable operation signal is gradually pulled down in compliance with the time constant to gradually lengthen the period Ton.
Right after the start, the maximum value of the period Ton gradually increases. After the passage of a predetermined period of time which complies with the operation of the DC--DC converter, however, the period Ton is set to a potential determined by the resistors R3 and R4. Thereafter, the Ton time becomes nearly constant though it may vary to a small degree depending upon the fluctuation in the output voltage.
As described above, the conventional DC--DC converter of the PWM control system is without problem while the input voltage Vin does not change much. When cells are used as an input power supply, however, the cell voltage gradually drops with the passage of time. When the input voltage Vin drops to such an extent that it is little different from the output voltage Vo, the maximum value of the output current decreases, too. Therefore, the output becomes insufficient before the cells are depleted, making it no longer possible to carry out the operation using the cells. So far, therefore, the input voltage had to be maintained at a level that is high to some extent. When the cell voltage drops below a predetermined value, the cells must be renewed.